Proto-oncogenes encoding growth factor receptors constitute several distinct families with close overall structural homology. The highest degree of homology is observed in their catalytic domains, essential for the intrinsic tyrosine kinase activity of these proteins. Examples of such receptor families include: the EGF-R and the related product of the erbB-2 oncogene; the Colony Stimulating Factor 1 receptor (CSF-1-R) and the related Platelet-Derived Growth Factor receptor (PDGF-R); the insulin receptor (IF-R) and the related Insulin-like Growth factor 1 receptor (IGF-1-R); and the receptors encoded by the related oncogenes eph and elk.
It is well established that growth factor receptors in several of these families play critical roles in regulation of normal growth and development. Recent studies in DrosophiIa have emphasized how critical and multifunctional are developmental processes mediated by ligand-receptor interactions. An increasing number of Drosophila mutants with often varying phenotypes have now been identified as being due to lesions in genes encoding such proteins. The genetic locus of the Drosophila EGF-R homologue, designated DER, has recently been identified as being allelic to the zygotic embryonic lethal faint little ball exhibiting a complex phenotype with deterioration of multiple tissue components of ectodermal origin. Furthermore, other mutants appear to lack DER function either in the egg or the surrounding maternal tissue. Thus, the DER receptor may play an important role in the ligand-receptor interaction between egg and follicle cells necessary for determination of correct shape of eggshell and embryo. It is not yet known whether DER represents the sole the DrosophiIa counterpart of both known mammalian erbB-related genes.
Some of these receptor molecules have been implicated in the neoplastic process as well. In particular, both the erbB and erbB-2 genes have been shown to be activated as oncogenes by mechanisms involving overexpression or mutations that constitutively activate the catalytic activity of their encoded receptor proteins (Bargmann, C. I., Hung, M. C. & Weinberg, R. A., 1986, Cell 45:649-657; Di Fiore, P. P., Pierce, J. H., Kraus, M. H., Segatto, O., King, C. R. & Aaronson, S. A., 1987, Science 237:178-182; Di Fiore, P. P., Pierce, J. H., Fleming, T. P., Hazan, R., Ullrich, A., King, C. R., Schlessinger, J. & Aaronson, S. A., 1987, Cell 51:1063-1070; Velu, T. J., Beguinot, L., Vass, W. C., Willingham, M. C., Merlino, G. T., Pastan, I. & Lowy, D. R., 1987, Science 238:1408-1410). Both erbB and erbB-2 have been causally implicated in human malignancy. erbB gene amplification or overexpression, or a combination of both, has been demonstrated in squamous cell carcinomas and glioblastomas (Libermann, T. A., Nusbaum, H. R., Razon, N., Kris, R., Lax, I., Soreq, H., Whittle, N., Waterfield, M. D., Ullrich, A. & Schlessinger, J., 1985, Nature 313:144-147). erbB-2 amplification and overexpression have been observed in human breast and ovarian carcinomas (King, C. R., Kraus, M. H. & Aaronson, S. A., 1985, Science 229:974-976; Slamon, D. J., Godolphin, W., Jones, L. A., Holt, J. A., Wong, S. G., Keith, D. E., Levin, W. J., Stuart, S. G., Udove, J., Ullrich, A. & Press, M. F., 1989, Science 244:707-712), and erbB-2 overexpression has been reported to be an important prognostic indicator of particularly aggressive tumors (Slamon, D. J., et al., 1989, supra). Yet, not all such tumors have been found to overexpress erbB-2, and many human tumors have not yet been associated with any known oncogene. Thus, there has been a continuing need to search for additional oncogenes which would provide knowledge and methods for diagnosis and, ultimately, for rational molecular therapy of human cancers.